This invention relates to an evaporation vessel of electroconductive ceramic substance having the shape of a boat and used for the vacuum evaporation of a metallic or non-metallic substance through resistance heating.
In the deposition of relatively large amounts of metallic or other substances by the vacuum evaporation method, boat-shaped evaporation vessels are generally used made of high-melting metals such as tungsten, tantalum and molybdenum or those made of electroconductive ceramic material introduced recently in the market.
FIG. 1 illustrates the conventional shape for a boat for evaporation vessels made of high-melting metals and FIG. 2 illustrates a conventional shape for evaporation vessels made of electroconductive ceramic material.
In the case of evaporation boats of these shapes, the molten metal in the course of vacuum evaporation may possibly creep out of the cavity portion denoted by the numeral 1 in FIG. 1 or in FIG. 2 to eventually reach the electrodes and, worse still, to result in the phenomenon of an undesirable formation of a short circuit. With a view to precluding such undesirable creeping of the molten metal, various devices have been proposed such as, for example, the insertion of notches at the portion adjoining the cavity (hereinafter referred to as the "limb portion") in tungsten boats. In one of the shapes of the electroconductive ceramic evaporation boats now available in the market, there has been adopted a device of forming the limb portion in a smaller thickness to raise the resistance as shown in FIG. 3 for thereby increasing the magnitude of heat generated at said limb portion and consequently preventing the molten metal from creeping up to the electrodes.
None of the devices so far proposed, however, have been able to completely eliminate the phenomenon of creeping of the molten metal out of the cavity portion. Even with the shape of the boat in FIG. 3, the molten metal can flow partially to the limb portion.
In the course of deposition, therefore, the resulting thickness in the deposited film is not uniform and the desired control of the film thickness is difficult to accomplish. Also in the case of repeated deposition, the distance over which the molten metal creeps out onto the limb portion is not consistent. For these reasons, evaporation boats of the shape being discussed cannot be used particularly where the deposited films are required to have a uniform thickness.